1. Field of the Invention
The present invention relates to a method of estimating a life of a ball screw for moving an injection screw included in an electric injection molding machine and generating an alarm or the like, and a life estimating system for carrying out the method.
2. Description of the Related Art
In an electric injection machine, an injection screw is reciprocated axially by rotating a ball screw by an electric motor. Therefore, the greatest load is exerted on the ball screw during an injection molding operation. If the ball screw is damaged during a long period of use, it is expected that the operation of the injection molding machine must instantly be stopped. If such a situation takes place, the broken ball screw must immediately be replaced with a new one, which interrupts the operation of the associated production line, adversely affecting the stability of the quality of moldings and may possibly cause damage in component parts associated with the ball screw.
To avid such a situation it is desired to estimate time when the condition of the ball screw is close to a condition that causes damage to the ball screw, i.e., to estimate the life of the ball screw, and to replace the ball screw with a new one before the same is broken.
Generally, it has been a general practice to decide the life of the ball screw by the intuition of an operator skilled in the operation of the injection molding machine and to change the ball screw when the operator decided that the life of the ball screw has almost terminated. It is difficult to estimate the life of the ball screw accurately only by the operator's intuition. The usable ball screw is thrown away and time and labor is wasted for the unnecessary change of the usable ball screw if the usable ball screw is changed too early. The ball screw will be broken and serious damage will be made to the associated component parts if the time for changing the ball screw is delayed excessively.
From such a point of view, a life estimating apparatus for estimating the life of a rotary member included in an injection molding machine or the like proposed in JP-A No. Hei 6-91683 is capable of accurately estimating, on the basis of a recognition that bearings supporting the ball screw is a most heavily loaded member, the life of a rotary member, such as a bearing or a ball screw.
This previously proposed life estimating apparatus comprises a revolution counter for directly or indirectly counting the number of revolutions of a rotary member driven for rotation by a driving motor, an arithmetic unit capable of calculating a fatigue life of the rotary member on the basis of the number of revolutions of the rotary member and torque required to rotate the rotary member, and a comparator for comparing the total number of revolutions of the rotary member counted by the revolution counter and fatigue life of the rotary member calculated by the arithmetic unit, and provides a warning when the total number of revolutions of the rotary member approaches or arrives at the fatigue life to prompt the operator to replace the rotary member with a new one.
The foregoing prior art life estimating apparatus for estimating the life of a rotary member included in an injection molding machine or the like compares a numerical value indicating the counted total number of revolutions of the rotary member, and a numerical value representing the fatigue life calculated on the basis of the torque of the driving motor required to drive the rotary member for rotation, and uses a condition where the difference between those numerical values is practically naught as a criterion for deciding the life of the rotary member. Particularly, an arithmetic procedure for calculating the fatigue life on the basis of the torque is very complicated and, if the life of the bearing is used for life estimation, many parameters in addition to the torque of the driving motor are necessary and the arithmetic procedure becomes further complicated.